The polymerization of isoprene to form a high cis-1,4-polyisoprene is well-known in the art. Under the usual or conventional conditions of polymerization of isoprene to form a high cis-1,4-polyisoprene, isoprene and an inert solvent, such as hexane and pentane are mixed together and polymerized with a catalyst system comprising a mixture of a trialkylaluminum compound, such as triisopropylaluminum, triethylaluminum, or triisobutylaluminum, or these trialkylaluminums complexed with an ether, such as diphenyl ether, anisole, or dibutylether, mixed with a cocatalyst which is usually a transition metal halide. Of particular interest in the formation of the high cis polymer is titanium tetrachloride. This produces a cement of polyisoprene which has a high cis-1,4-content. Under these polymerization conditions the polymerization is continued until the desired conversion is reached and then the catalyst system is usually destroyed by the addition of an amine compound, such as tetraethylene pentamine (TEPA) or triisopropanol amine (TIPA), or the like.
The function of these amines, which will be called shortstops or shortstoppers in this application, is to destroy the catalyst system and when the synthetic polyisoprene is mixed with an antioxidant to cause the polyisoprene to become somewhat more stabilized upon storage. In the past, high cis 1,4-polyisoprene which has been shortstopped with TIPA produces a polymer which is low in color, that is the color of almost water-white. On the other hand, when a high cis-1,4-polyisoprene is shortstopped with TEPA, a polymer is produced which is yellow in color, but has better dynamic properties when compared with a polyisoprene which has been shortstopped with TIPA.
The desirable features of a finished polymer of high cis-1,4-polyisoprene should include a light color approaching whiteness and the ability of the polymer to retain this light color, not only after prolonged exposure of the raw rubber to ultra-violet light and particularly, sunlight, but also during and after the critical conditions encountered in compounding, processing and curing of these rubber compounds.
The present invention is directed to a new shortstop to stop the polymerization and produce in the polyisoprene a low color polymer which has the desired dynamic properties in the cured state, when employed in any such applications as tires. In other words, the use of the shortstop of this invention provides a synthetic polyisoprene with the combined features of both the conventional shortstops, TIPA and TEPA. In addition, it has been found that the use of the shortstop of this invention provides an improved green strength and better Mooney stability on storage. PG,4